1. Field of the Invention
The present invention relates to a fluid-filled cylindrical elastic mount for flexibly connecting two or mounting one of the two members on the other member, in a vibration-damping manner. More particularly, the present invention is concerned with such a fluid-filled cylindrical elastic mount suitable as an engine mount for a motor vehicle, for example, which is capable of effectively damping or isolating vibrations over a wide frequency range.
2. Discussion of the Prior Art
As a vibration damper interposed between two members of a vibration system, for flexibly connecting the two members or flexibly mounting one of the two members on the other member, there is known a so-called fluid-filled cylindrical elastic mount as disclosed in JP-A-56-164242 and JP-A-63-289349. The elastic mount as disclosed in these publications has an inner sleeve, an outer sleeve radially outwardly spaced from the inner sleeve, and an elastic body interposed between the inner and outer sleeves for elastically connecting these sleeves. The outer sleeve and the elastic body cooperate to define a first and a second fluid chamber which communicate with each other through an orifice passage.
The fluid-filled cylindrical elastic mount of the type indicated above exhibits excellent vibration damping characteristics based on the resonance of a non-compressible fluid flowing through the orifice passage between the two fluid chambers, upon application of a vibrational load between the inner and outer sleeves. The damping characteristics exhibited by this type of elastic mount are superior to those exhibited by an elastic mount which does not contain a non-compressible fluid and which relies solely on the elasticity of an elastic body. The fluid-filled cylindrical elastic mount is typically used as an engine mount, a differential mount and a suspension bushing for a motor vehicle.
However, the known fluid-filled cylindrical elastic mount of the above type provides a sufficiently high damping effect with respect to a certain frequency range of the input vibrations. That is, the damping of the input vibrations based on the resonance of the fluid flowing through the orifice passage is effective only to a predetermined frequency band to which the orifice passage is tuned. When the frequency of the input vibration is higher than the predetermined frequency band, the amount of the fluid flow through the orifice passage is almost negligible as if the orifice passage were closed, and the elastic mount is not capable of exhibiting a sufficiently high damping effect with respect to the higher frequencies.
In view of the drawback experienced on the known fluid-filled cylindrical elastic mount, the assignee of the present application developed a fluid-filled elastic mount having two independent orifice passages which extend between two fluid chambers, as disclosed in JP-B-2-5937. In this elastic mount, the ratio of the cross sectional area to the length of the second orifice passage is made higher than that of the first orifice passage, and a movable plate is disposed in the second orifice passage, so as to restrict the amount of flow of the non-compressible fluid through the second orifice passage.
In the elastic mount of the type indicated just above, the movable plate is displaced to cause fluid flow through the second orifice passage upon application of vibrations of relatively high frequencies to which the first orifice passage is not effective to damp. Consequently, the elastic mount provides a high damping effect with respect to such high-frequency vibrations, based on the resonance of the fluid flowing through the second orifice passage. Therefore, this type of elastic mount is capable of effectively damping or isolating the input vibrations over a relatively wide frequency range.
A further study by the present inventors on the known fluid-filled cylindrical elastic mount having two orifice passages revealed a drawback which arises from the provision of the two mutually independent orifice passages and the movable plate disposed in the second orifice passage. That is, the elastic mount requires a complicated structure for defining the two orifice passages and movably supporting the movable plate. The structure uses a relatively large number of components, and requires a cumbersome procedure for assembling the components.